Ironwood is a small, slow-growing pyramidal tree native to the dry understory in our local oak woods. Its small mature size is great for restricted spaces and the tree will thrive in sun or heavy shade. While it has shown promise as a street tree, it is intolerant of salt and heavy air pollution. In fall, persistent hop-like seeds adorn the finely textured canopy to give it four-season interest. Alternative common names include Eastern Hophornbeam, American Hophornbeam, and Leverwood.
Wisconsin Native: Yes
USDA Hardiness Zone: to zone 3
Mature Height: 25 feet
Mature Spread: 15 feet
Growth Rate: Slow
Growth Form: Upright pyramidal tree
Light Requirements: Full Sun to Full Shade
Site Requirements: Tolerant of many sites, intolerant of flooding
Flower: Monoecious, catkins
Bloom Period: mid-May to mid-June
Foliage: Dark Green
Fall Color: Golden Yellow
Urban Approved: No
Fruit Notes: Hop-like strobili, green to brown, showy, persistent
Waukesha County, WI
Ironwood excels as a small-stature ornamental where flowering is not desired. It provides tremendous interest, even in shaded locations. Single-stem trees can be used as a specimen or component of a shade garden, while multistem trees can be made a component of shrub borders or used as a unique ornamental in a restricted space.
As it is intolerant of pollution and road salt, Ironwood should not be sited near busy roadways or heavy industry. However, it has grown successfully as a street tree in Green Bay, Wisconsin.
Occasionally the young branches or seedlings of Ironwood can be browsed by deer, although this damage is rare and only seen when other, more preferable food sources are exhausted.
Ironwood is an excellent food source for Ruffed Grouse and is of equal importance as Aspen and Birch. Spring catkins and buds are high quality forage for grouse and an important habitat component.
Wild Turkey, Squirrel, Cottontails, Ring-Necked Pheasant, Rose Breasted Grosbeak, Downy Woodpecker, and Purple Finch have been observed to feed on the hop-like seeds and buds. Birds also act as a secondary method of seed dispersal for Ironwood, allowing it to colonize (though not aggressively) partially shaded woodlands.
Like all trees, Ironwood will benefit from a 2 inch thick mulch ring around the rooting area.
Newly planted Ironwood trees will benefit from a systemic insecticide application to prevent Two Lined Chestnut Borer damage. This insect is only problematic when the tree is stressed from transplanting, so there is no need for supplemental applications.
If the trunk suffers animal or mechanical damage, Ironwood will stump-sprout. This can be avoided by using a trunk guard in the winter and maintaining a wide mulch ring around the tree.
Stressed Ironwood trees (especially when transplanting) are susceptible to attack by Two-Lined Chestnut Borer. Newly planted trees should be given a systemic insecticide to prevent insect damage. The best way to avoid this pest is to avoid stressing the tree through proper mulch and watering practices.
Trunk and Butt Rot are most significant problems from a forestry standpoint, but these diseases impact timber quality and have little impact in a landscape setting.
The male catkins of Ironwood develop in late summer but do not disperse pollen until the following spring. Female catkins develop in spring as foliage starts to emerge, with peak pollen dispersal occurring around one month later. Good seed production begins once the tree has reached 25 years of age. The seeds are light- one pound can contain upwards of 30,000 seeds! The hop-like fruits emerge green, aging to brown. In fall the fruits shatter, dispersing the seeds.
Again, Ironwood is Monoecious, meaning it has male and female parts on the same plant. In late summer, male catkins form and remain on the tree through winter waiting for the female catkins to arrive next spring. When the female catkins arrive in spring, the male and female catkins pollinate (primarily by wind pollination) between mid-May to mid-June.
As a result of the pollinating, hop-like fruits are borne typically in late-summer. The fruits emerge green then age to brown until fall when they shatter, dispersing the seed to grow new Ironwood trees. NOTE: Don’t worry about small Ironwood trees showing up in your yard. Ironwood seed germination is a complicated, nearly 2-year process. Wildlife will grab the seeds long before they can germinate in a formal, turf-lawn site. Seed germination is more likely to occur in a natural site, however, Ironwood is a slow growing tree.
Ironwood is found across the eastern United States. Its original range was restricted by fire, with Ironwood typically found in ravines and bottomlands along rivers, or on exposed bluffs where fire is less prevalent. While the tree will respond to light fire by resprouting, heavy fires will kill existing trees as well as the seedbed. Aside from fire, the species is quite adaptable and able to colonize a variety of sites, ranging from open fields with heavy grass competition to open woodlots.
Ironwood is an intermediate species that establishes in open woodlands, occupying the midstory of a forest canopy. While Sugar Maple and Red Oak are the dominant overstory, Ironwood fills the space between mature trees and understory shrubs. In silviculture, Ironwood is sometimes viewed as a ‘weed tree’ due to its low timber value, smaller mature size, and its ability to compete with more high-value species like Sugar Maple.
Compared to to other trees in the same sites, the foliage of Ironwoods tends to have higher calcium content.
With a name like Ironwood, one would assume the wood is strong, durable, and long lasting. However, the wood has terrible resistance to rot and should not be used in contact with soil or moisture. Its best use is for tool handles, hence the name Leverwood.
Although it is a fine-textured tree, Ironwood is not prone to damage from heavy snow loads or cold.
Natural companions of Ironwood include Aspen, White Pine, and Oak in early-successional forests, and Hemlock, Sugar Maple, Beech, and Yellow Birch in late-successional forests. Understory associates include American Hazelnut, Dwarf Bushhoneysuckle, Common Witchhazel, and American Elderberry.